A. Technical Field
The present invention relates to a catalyst for purification of exhaust gases and a process for purification of exhaust gases. Specifically, the present invention aims at: a catalyst for purification of exhaust gases to efficiently purify carbon monoxide (CO) as contained in combustion exhaust gases as discharged from various combustion apparatuses such as boilers, gas turbines, diesel engines, and gas engines; and a process for purification of exhaust gases utilizing such a catalyst.
B. Background Art
The combustion exhaust gases, as discharged from various combustion apparatuses such as boilers, gas turbines, diesel engines, and gas engines, generally contain harmful components such as CO, NOx, SOx, and unburned-fuel-derived volatile organic compounds, though depending on such as their respective combustion apparatuses and operational conditions. As to these combustion apparatuses, there are many cases where combustion is carried out under conditions where the quantity of air as supplied during combustion is rendered larger than the theoretical air quantity necessary for perfect combustion of fuel gases in order that the combustion efficiency or thermal efficiency may be enhanced and further that the quantities of harmful components such as CO and NOx may efficiently be decreased. The quantities of the harmful components such as CO and NOx as contained in the combustion exhaust gases can considerably be decreased by such as the above control of the combustion state, but such as CO and NOx still remain on a level recognized as harmful. Therefore, these residual harmful components need to also efficiently be purified, so it becomes necessary to develop a catalyst for purification of exhaust gases and a process for purification of exhaust gases which are able to function effectively on a higher level of purification.
By the way, known as a catalyst for purification of exhaust gases is a catalyst comprising a metal oxide as a support and a catalytic component supported thereon wherein the catalytic component includes a noble metal element.
Examples of such a catalyst for purification of exhaust gases, comprising the metal-oxide-supported noble metal, include: those which have a structure as molded monolithically in the shape of such as a plate or honeycomb; and those which have a structure as supported on another framework structure including such as metals and/or cordierite; and such a catalyst is placed in a passageway of the exhaust gases. The exhaust gases flowing through comes into contact with the catalyst, so that harmful substances in the exhaust gases are catalyzed and thereby converted into harmless substances or into substances easy to make harmless or remove by post-treatment.
The catalyst for purification of exhaust gases, comprising the metal-oxide-supported noble metal, can display complex catalysis between the metal oxide and the noble metal and is therefore advantageous in that: when compared with catalysts for purification of exhaust gases comprising metal oxides alone, the harmful components in the exhaust gases can be purified with high efficiency, and the range of the kinds of harmful components possible to purify extends also. For example, JP-A-146991/1978 discloses catalysts for purification of exhaust gases comprising: binary composite oxides (e.g. Ti—Si-composite oxide) or trinary composite oxides; and noble metals such as Pt and Pd as supported on these composite oxides.
However, for the purpose of efficiently purifying the aforementioned residual harmful components, conventional catalysts for purification of exhaust gases comprising the metal-oxide-supported noble metal may be still unable to sufficiently display performance and function of demanded high levels. Thus, if usual, as to the metal-oxide-supported noble metal, if the quantity of the noble metal as supported is increased, the catalytic function is expected to be enhanced, but actually there are problems in that: if the quantity of the supported noble metal with a high unit cost is increased, the cost of materials as a whole greatly rises correspondingly thereto, so the economical performance is inferior, and besides, the enhancement of the purifiability on a high level is seen insufficiently for high cost. Furthermore, as to SOx often contained as harmful components in the exhaust gases, there are problems in that: there is seen a phenomenon such that, as the quantity of the supported noble metal in the catalyst is increased, the conversion of SO2→SO3 rather becomes higher, so SO3 which causes such as corrosion of piping is formed in a large quantity.